some-dict-of
Have you ever needed an existential wrapper that just guaranteed class
membership, but you didn’t need to know the specifics of the type in question?
Well, you probably don’t - it’s not idiomatic Haskell for the most part. But
sometimes this comes up, and I wanted a nice packaging of the technique. I wrote
this mostly to support the discover-instances
library.
SomeDictOf
Let’s consider a type that wraps anything that can be shown, along with a value
of that type. We can make one by hand like this:
data SomeShowable where
SomeShowable :: forall a. Show a => a -> SomeShowable
showList :: [SomeShowable] -> [String]
showList = map (\(SomeShowable a) -> show a)
With SomeDictOf
, we can generalize this pattern to other classes and even
other containers.
someShowable :: SomeDictOf Identity Show
someShowable =
SomeDictOf (Identity (3 :: Int))
We’re carrying an Int
, but the type does not reveal this. We can create a list
of values like this, and we can call show
on them.
showValues
:: [SomeDictOf Identity Show]
-> [String]
showValues =
map (\(SomeDictOf (Identity showable)) -> show showable)
We can also carry around evidence of a type class instance, and then write
generic stuff based on it. Consider the
PersistEntity
from the persistent
database library.
tables :: [SomeDictOf Proxy PersistEntity]
tables = [ SomeDictOf (Proxy @User), SomeDictOf (Proxy @Organization) ]
Now we can iterate over these types and, say, load all the rows out of the
database and verify they parse.
checkRows :: SqlPersistT [[PersistValue]]
checkRows = do
forM tables $ \(SomeDictOf (Proxy :: Proxy table)) -> do
results <- selectList [] [] :: SqlPersistT m [Entity table]
pure (map toPersistValue results)
Or we can do some metaprogramming based on their EntityDef
s, since
PersistEntity
has a method Proxy a -> EntityDef
.
getDefinitions :: [EntityDef]
getDefinitions =
map (\(SomeDictOf p) -> entityDef p) tables